Redox‐Polysilsesquioxane Film as a New Chloride Storage Electrode for Desalination Batteries

Abstract

Desalination battery is an emerging concept used in electrochemical technology to solve the problems in sea water desalination. However, identifying an efficient and reversible chloride (Cl−) storage electrode is a major challenge for the development of desalination batteries. A new membraneless, reversible desalination battery consisting of an efficient, reversible, high‐capacity Cl− storage electrode, ferrocyanide molecule immobilized polysilsesquioxane (redox‐PSQ), in combination with the cation storage material, a nickel hexacyanoferrate film in real sea water, is described. The redox‐PSQ polymer is superior over the other reported Cl− storage electrodes (Ag and Bi electrodes) in terms of cost, stability, reversibility, and Cl− ion removal capacity in real‐time applications. About 164 mg L−1 of Cl− ion is removed from natural sea water per 7.6 × 10−8 mol cm−2 surface concentration of electrode material with 98% coulombic efficiency. During the desalination process, alkali ions move toward nickel hexacyanoferrate and Cl− ions moves toward the redox‐PSQ film from the sea water. During the salination process, the stored ions are recovered from the electrode surface as a result of partial energy recovery. Furthermore, this electrode also removes the sulfate ion present in the sea water by the electrostatic adsorption phenomenon. In addition, a high‐voltage (2.3 V), Mg//redox‐PSQ polymer cell is constructed with a red light‐emitting diode that glows during the salination process.

Faradaic Electrodes Open a New Era for Capacitive Deionization

“…Similar strategies were also conducted for self-doped polypyrrole with ionizable sodium sulfonate [282] and polyaniline with selfdoped benzenesulfonic anions. [283] Another case was demonstrated by Silambarasan et al, [44] who introduced a redox (Figure 26f,g). As a result, the [Fe(CN) 6 ] 4 -PSQ based CDI cell, in combination with a NiHCF film as the Na + capture material, achieved a superior desalination performance.…”

“…Such charge transfer can be accomplished by ion insertion within crystal structures, [40,41] conversion reaction with forming new compounds, [42,43] or ion-redox active moiety interaction. [44,45] This endows Faradaic materials with three advantages over carbon as CDI electrode. First, Faradaic reactions within the bulk of electrodes allow for higher desalination capacity, which can be as high as >100 mg g −1 .…”

“…[50][51][52][53] Apart from exclusive Na + /Cl − capture materials, some materials have emerged for both Na + and Cl − capture, such as MXenes [54] and transition metal dichalcogenides [55] consisting of 2D layers (with large interlayer distance) relying on the mechanism of ion insertion. In addition, redox-active polymers are also promising for water desalination, as they can show either strong interactions with Na + (such as redox-active polyimide) [56] or Cl − (such as polymers with [Fe(CN) 6 ] 4− ) [44] depending on the tunable redox active moieties. In addition to the aforementioned novel ion-selective Na + /Cl − capture electrode materials [47,57,58] and various cell designs, [43,46,49] some recent promising developments of Faradaic electrodes have been achieved, such as research on the influences of operational parameters on CDI performance metrics, [41,43,59] and some typical scientific and practical application.…”

“…− group for Na + capture), [140] or doping with reactive inorganic ions (such as [Fe(CN) 6 ] 4− for Cl − capture). [44] Details for the ion-redox active moiety interaction of polymers will be specified in Section 3.3.3. Comparatively, this mechanism is more kinetically facile than the conversion mechanism, because no new compound forms and structural evolution does not occur in the ion-redox active moiety interaction.…”

Functional materials in desalination: A review

“…Salt rejection through desalination batteries make them applicable for treating high salinity feeds, providing a clear advantage over the RO technology. Several other studies report on similar, advanced desalination batteries providing a cost-effective seawater desalination option through utilization of electrochemical redox reactions[150,151].…”